Comparison of acidity and chemical composition of summertime cloud water and aerosol at an alpine site in Northwest China: Implications for the neutral property of clouds in the free troposphere

Aerosol and cloud acidity are essential to human health, ecosystem health and productivity, as well as climate effects. The main chemical composition of cloud water greatly varies in different regions, resulting in substantial differences in the pH of cloud water. However, the influences of the anthropogenic emissions of acidic gases and substances, alkaline dust components, and dicarboxylic acids (diacids) on the ground concerning the acidity of cloud water in the free troposphere of the Guanzhong Plain, China, remain clear. In this study, cloud water and PM_2.5 samples were simultaneously collected in the troposphere (Mt. Hua, 2060 m a.s.l). The results indicated that the cloud water was alkaline (pH = 7.6) and PM_2.5 was acidic (pH = 3.2). These results showed the neutral property of clouds collected in the heavily polluted Guanzhong Plain, although most previous studies always considered acidity as a marker of pollution. The sulfate (SO₄ ²⁻), nitrate (NO₃ ⁻), and ammonium (NH₄ ⁺) (SNA) of particulate matter and cloud water in the same period were compared. SO₄ ²⁻ was dominant in particulate matters (accounting for 63.4 % of the total SNA) but substantially decreased in cloud water (only 30.1 % of the total SNA), whereas NO₃ ⁻ and NH₄ ⁺ increased from 28.5 % and 8.2 % to 39.8 % and 30.2 %, respectively. This could be attributed to the complex formation mechanism and sources of SO₄ ²⁻ and NO₃ ⁻ in the cloud. The results of ion balance indicated that a significant deficit of inorganic anion equivalents was observed in the cloud water samples. The high concentration of diacids in the cloud phase (1237.4 μg L⁻¹) may facilitate the formation of salt complexes with NH₄ ⁺, thus influencing the acidity of the cloud water. The pH of cloud water has increased in recent decades due to the sustained reduction of sulfur dioxide, which may also affect the acidity of future precipitation.